This invention relates to a method for converting radiographic image, a radiation energy storage panel having a stimulable phosphor-containing layer and a phosphor, more particularly, it is concerned with a method for converting radiographic image utilizing a stimulable phosphor, a radiation energy storage panel utilizing an alkali halide phosphor activated with thallium and the like, and an alkali halide stimulable phosphor.
Hitherto, there has been employed the so-called radiography using a silver salt for a radiation image, but there has now been desired a method for making a radiation image without using any silver salt.
Instead of the aforesaid radiography, there has been reviewed a method wherein radiation transmitted through the subject is absorbed in a phosphor, the phosphor is excited with a center energy to emit a radiation energy having accumulated in said phosphor as fluorescence and the fluorescence is detected to form an image. Illustratively, there has been proposed a method wherein a thermostimulable phosphor is used as a phosphor and a radiation image is converted by using a thermal energy as an exciting energy [see British Patent No. 1,462,769 and Japanese Provisional Patent Publication No. 29889/1976]. This conversion method is to employ a panel having a thermostimulable phosphor layer formed over a support, absorb a radiation transmitted through the subject into the thermostimulable phosphor layer of said panel so as to accumulate radiation energy upon strength and weakness of radiation, heat the thermostimulable phosphor layer to take out the accumulated radiation energy as a photosignal and then form an image upon strength and weakness of the luminescence. However, this method required heating for converting radiation energy to a photosignal and thus needed absolutely that the panel has a heat resistance without any modification or deformation by heat. Thus, there was a great restriction to a thermostimulable phosphor layer forming a panel, a raw material for a support and the like. There is thus seen a great difficulty in practical application with regard to this method for converting radiographic image using a thermostimulable phosphor as a phosphor and a thermal energy as an exciting energy. On the other hand, there has been also known another method for converting radiographic image wherein one uses a panel having a stimulable phosphor-containing layer placed over a support and as an exciting energy either or both of visible light and infrared light (see U.S. Pat. No. 3,859,527). This latter method may be said to be a more preferable method for converting radiographic image, in view of no heating required for converting the accumulated radiation energy to a photosignal as done in the former method and no need for heat resistance of a panel.
Among the phosphors employed in the aforesaid method for converting radiographic image, there have been known as a thermostimulable phosphor such phosphors as LiF:Mg, BaSO.sub.4 :Mn, CaF.sub.2 :Dy and the like. Moreover, there have been known such phosphors as KCl:Tl or BaFX:Eu.sup.2+ type (X: Cl, Br, I) phosphor as disclosed in Japanese Provisional Patent Publication No. 75200/1984 as a stimulable phosphor using as an exciting energy a visible light or an infrared light.
Now, where the said method for converting radiographic image is to be applied to X-ray image conversion for medical diagnosis, the method is desirably of a high sensitivity as far as possible in order to reduce an exposed dose for patients, so that it is desired that the stimulable phosphor used for the method may show a high emmision luminance by stimulation as far as possible.
In the above-mentioned method, a reading speed for radiation image should be made higher for enhanced operation efficiency as a system and, therefore, a stimulable phosphor may desirably have a rapid response speed of stimulable emission to an exciting light.
In the above-mentioned method, a radiation energy storage panel is repeatedly used after elimination of the residual image formed by previous use; however, it is desirable that an elimination time for residual image should be short in said radiation energy storage panel and also that the stimulable phosphor used in the method should have a rapid elimination time for residual image.
However, the above stimulable phosphors are not completely satisfactory in all respects to stimulable emission luminance, response speed to stimulable emission and elimination speed for residual image and there has been desired an improvement in them.
Additionally, a reading apparatus for taking radiation image in the above methods should be desirably miniaturized, of a low cost and simplified and, thus, it is essential to use as an exciting light source a semi-conductor laser rather than a gas laser, e.g., an Ar.sup.+ laser, an He-Ne laser and the like. Accordingly, it is desirable that the stimulable phosphor employed in this method should desirably have a stimulable exciting spectrum adaptable to an oscillating wave length (not less than 750 nm) of a semi-conductor laser.
However, the above stimulable phosphor does hardly exert a stimulable emmision to an oscillating wave length of a semi-conductor laser and thus there has been desired a longer wave length of a stimulable exciting spectrum.
As the alkali halide phosphor, there have been also known CsI:Na, CsI:Tl, CsBr:Tl, RbBr:Eu, RbCl:Eu, KCl:Tl, LiF:Mg and the like. Among them, CsI:Na or CsI:Tl has been applied to I.I. tube for X-ray, while CsBr:Tl has been tried for application to a similar use. Also, it is known that RbBr:Eu, RbCl:Eu or LiF:Mg be a thermostimulable phosphor and that KCl:Tl may show a stimulable phenomenon.
It was seen that a stimulable phosphor may be utilized as an accumulated radiation energy storage panel by absorbing radiation transmitted through the subject, emitting as fluorescence the radiation energy having accumulated in the phosphor by irradiation with either or both of visible light with a longer wave length and infrared light and detecting said fluorescence to form a radiation image of the subject. But, when applied as such a radiation energy storage panel, the subjects may be frequently human beings, therefore, there is needed a less exposed dose to the subject as far as possible and there is desired as the phosphor therefor a phosphor having a higher stimulable emission efficiency. Further, a scanning time per one image element is practically about 10 usec from the relationship with a reading time and a resolving power and a readable area in a radiation energy storage panel and then there is desired a phosphor having a shorter shelf life of stimulable emission. And further, the afterglow from stimulable emission on reading, if any, may cause deterioration of a SN ratio and then there is desired a phosphor showing no such an afterglow phenomenon.
In the prior art, for obtaining a radiation image, the so called radiation photographic method by use of a silver salt has been utilized, but it has become desired to have a method for converting the radiation image into an image without use of a silver salt.
As a method substituting for the above radiation photographic method, there has been proposed the method in which the radiation transmitted through a subject is absorbed onto a phosphor, then the phosphor is excited with a certain kind of energy to radiate the radiation energy accumulated in the phosphor as luminescence, which luminescence is detected to form an image. As a specific method, there has been known a radiation image converting method wherein a panel having a stimulable phosphor layer formed on a support is used and one or both of visible ray and infrared ray is used as the excitation energy (U.S. Pat. No. 3,859,527).
The performances demanded for the stimulable phosphor to be used in the above radiation image converting method may include high intensity of stimulated luminescence so that the dose exposed may be smaller, rapid response speed of stimulated luminescence to the stimulating ray so that high speed reading may be possible, easiness in cancellation of remained images, and stimulation spectrum adaped to the oscillation wavelength of semiconductor laser (750 nm or longer) which can make the reading device small scale and low cost.
As a radiation image converting method by use of the stimulable phosphor satisfying these requisite performances, there is the radiation image converting method by use of an alkali halide phosphor as disclosed in Japanese Unexamined Patent Publication No. 72088/1986, etc.
In the above radiation image converting method, the operation to obtain a radiation image of a subject is performed by absorbing the radiation energy transmitted through the subject or emitted from the subject onto the stimulable phosphor constituting the radiographic image storage panel, scanning the radiographic image storage panel containing said stimulable phosphor with a laser beam to release the radiation energy accumulated as luminescence in time series, and detecting the luminescence
Whereas, in a phosphor, there is generally observed luminescence emitted continuously even after stopping excitation, namely a phenomenon of afterglow, and such afterglow of stimulated luminescence is also seen similarly in the stimulable phosphor to be used for the radiographic image storage panel. While the stimulated luminescence to be detected is emitted from the picture element irradiated with the stimulating ray at a certain time point, afterglow of stimulated luminescence is emitted from all the picture elements scanned with the stimulating ray before said time point, and a part thereof is detected mixed with the stimulated luminescence to be detected, whereby the image obtained by the radiographic image storage panel containing such phosphor will be lowered in image quality.
The above trouble of afterglow of stimulated luminescence is improved in the alkali halide phosphor disclosed in Japanese Unexamined Patent Publications No. 73786/1986 and 73787/1986, but it cannot be said to be satisfactory yet, and it would be desirable to further improve the afterglow of stimulated luminescence characteristic of the alkali halide phosphor.